As is known in the art, there is a desire for wideband, high-power (>25 dBm) silicon based amplifiers in microwave systems. However, high-speed silicon-based technologies typically incorporate CMOS or HBT devices with modest breakdown voltage levels (BVCEO <4 V), which results in high operating currents in the amplifier core. These high dc and ac currents used in the amplifier core require large width metal routing to satisfy electromigration concerns and result in lossy passive structures. The ability to distribute the dc current amongst various amplifier stages and de-couple the dc amplifier current from the ac output current enables wideband, high output power silicon-based amplifier designs
As is also known in the art, there are numerous circuit topologies used in silicon-based power amplifiers. Most designs consist of a cascode structure, using bipolar, silicon-germanium heterojunction bipolar, or CMOS transistors for the transistors, with standard L-C matching structures to deliver the maximum output power for a given source and load impedance as shown in FIG. 1. Depending on the application, packaging, and system requirements, the design can be single-ended or differential, with various biasing schemes (i.e. class A, AB, B, E, F, etc.) for the desired linearity, efficiency, and gain. In all of these design topologies the circuit is relatively narrowband, due to the resonant behavior of the matching structures.
As is also known in the art, distributed amplifiers have been shown in silicon technologies, but are generally designed in a single-ended fashion. Distributed amplifiers increase the gain-bandwidth product of a given amplifier stage by connecting several amplifier stages in series via transmission line elements. The design can also be realized by using discrete inductor and capacitors to act as artificial transmission line elements, as seen in FIG. 2. The signal propagates down the input of the distributed amplifier, being amplified by each discrete transistor, until it reaches the termination resistor. The output of each discrete transistor will then be combined in the collector output network (assuming the phase velocities of the input and output L-C networks are identical) to create the final broadband output.
These designs achieve a wideband of operation, but at modest output power levels (˜20 dBm) over the band of operation. This output power limitation is typically due to the implementation of these amplifiers, where all of the output current (or collector current) must flow through each matching inductor. This current includes the dc current for each device, resulting in significantly high dc current flowing through the inductors located near the output. In order to accommodate this large dc current, the inductors near the output must be very wide to avoid electromigration concerns. Obviously a dc blocking capacitor could be inserted between the stages, but this would then require an additional biasing inductor at the collector of each stage, which would also degrade the circuit's performance.
As is also known in the art, the use of incorporating transformer coupled silicon-base power amplifiers has also been demonstrated in numerous works, where the output match of several amplifiers is combined via transformer elements on the silicon die. Transformer coupled amplifiers, or amplifiers having spatially distributed transformers, use monolithic transformer structures (typically intertwined inductors) to combine the output of several discrete amplifiers, as shown in FIG. 3. In this case, the input signal is split evenly amongst the amplifying transistors, with each transistor receiving the same phase and amplitude. The output of each transistor will also have the same magnitude and phase, allowing them to be summed coherently. This summation of signals will result in a higher output power for the entire amplifier than what could be achieved with a single amplifying element.
Although this topology does enable higher output power, it still maintains a narrow band frequency response. Since all of the inputs and output of the circuit are in-phase and have identical matching structures, the narrow-band shape of the transfer function will also be identical resulting in an overall narrow band response.
Further, the concept of incorporating transformer coupled silicon-base power amplifiers has also been demonstrated in numerous works: P. Haldi, D. Chowdhury, P. Reynaert, L. Gang, and A. Niknejad, “A 5.8 GHz 1 V Linear Power Amplifier Using a Novel On-Chip Transformer Power Combiner in Standard 90 nm CMOS,” IEEE Journal of Solid State Circuits, vol. 43, no. 5, pp. 1054-1063, May 2008; I. Aoki, S. D. Kee, D. B Rutledge, and A. Hajimiri, “Distributed active transformer-a new power-combining and impedance-transformation technique,” IEEE Transactions on Microwave Theory and Techniques, Vol. 50, pp. 316-331, January 2002 where the output match of several amplifiers is combined via transformer elements on the silicon die. Although this topology does enable higher output power, it still maintains a narrow band frequency response. Since all of the inputs and output of the circuit are in-phase and have identical matching structures, the narrow-band shape of the transfer function will also be identical resulting in an overall narrow band response.
Distributed amplifiers have also been shown in silicon technologies, but are generally designed in a single-ended fashion, as demonstrated in B. Sewiolo, D. Kissinger, G. Fischer, and R. Weigel, “A High-Gain High-Linearity Distributed Amplifier for Ultra-Wideband-Applications Using a Low Cost SiGe BiCMOS Technology,” IEEE 10th Annual Wireless and Microwave Technology Conference, 2009, pp. 1-4, 2009. These designs achieve a wideband of operation, but at modest output power levels (˜20 dBm) over the band of operation. This output power limitation is typically due to the implementation of these amplifiers. As seen in FIG. 2, all of the output current (or collector current) must flow through each matching inductor. This current includes the dc current for each device, resulting in significantly high dc current flowing through the inductors located near the output. In order to accommodate this large dc current, the inductors near the output must be very wide to avoid electromigration concerns. Obviously a dc blocking capacitor could be inserted between the stages, but this would then require an additional biasing inductor at the collector of each stage, which would also degrade the circuit's performance.